Introduction

This page describes how I solved the THX subwoofer crossover frequency issues while using a Marantz AV-9000 THX Ultra preamp in my home theatre stereo system with my own original subwoofer design, by adding a new low pass filter with a front end mixer.  It works by connecting all of the front channels from the Marantz AV-9000, left, right, center and bass via splitter cables to the four inputs of the mixer on the new filter.  This enables the subwoofer to operate with a single full range mono channel that can be filtered as required with out regard for anything that may have been done to the signal within the DSP processing of the Marantz AV-9000.

Rev B Active Filter

The main new requirement of the Rev B version of the active filter is to provide a four channel mixer at the input as described above.  To enable this I have changed the design as shown in the diagram below.  The first stage (section A in the diagram) is a four channel inverting buffer & mixer with a 47Kohm input impedance.  This impedance gives a more even balance with the inputs of the power amps that will be connected in parallel from the preamp output for the various channels, whereas the 3k9 input impedance of the Rev A version was exclusively connected to a single preamp output.  The outputs of the mixer channels are connected together and then connect to the input of the second stage (section B in the diagram) which is the low pass Sallen & Key filter.  This arrangement is better that the Rev A version because it means that the filter is working with the full line level input rather than a possibly reduced level from the gain stage which will improve the signal to noise ratio.  Rather than using cabled rotary switches for the filter settings, I used individual DIL switches soldered directly to the board.  This has the advantage of being able to close multiple switches at the same time to make much bigger capacitor values for very low crossover frequencies and to mess around with the relationship of C1 to C2 to give different Q factors which will result in different responses other than Butterworth etc.  Also it is much quieter as there are no cables to pickup any noise.  Of course there is the obvious disadvantage that it is not easy to change the settings, but once the setting is found, you will never ever need to change it again for any given setup.  This means there will be no electrical noise from the switches over time and also that there is no need to worry about making the seal where the switch passes through the cabinet airtight.  When I was setting up, I just had some temporary long cables to connect the filter board so that it could hang out of one of the bass reflex ports to give access.  When the setup was done, I fitted it into it's final place forever.  If you want to, you could still add some other kind of switch with external access if needed.  The third and final stage is the variable gain inverting buffer (section C in the diagram) that has the gain control and connects directly to the power amp input.

2nd order Butterworth Sallen & Key cut off frequency formulaThe table below shows a selection of switch settings for various crossover frequencies from 12Hz to 122Hz for a Butterworth response where R1 = R2 and C1 = 2 x C2. The mathematics that describe the filter are shown here.  The place in the table where the frequency goes in the order of 20, 22, 21 is correct, its just that with the capacitor values getting larger, the change in resistor value actually overlaps the capacitor values a bit.  For the very large capacitor values at very low frequencies, there is no value in changing the resistor value from 548K at all!  If you want to, you can use the formula shown to workout any combinations not shown, but remember this formula only works for a Butterworth response where R1 = R2 and C1 = 2 x C2.  The overall response of my particular subwoofer with the crossover frequency set to 17Hz is described here.  With this modification to my subwoofer, it now works just fine with THX!  The complete circuit and layout of the new board is shown here.

S1/S9 S2/S10 S3/S11 S4/S12 S5/S13 S6/S14 S7/S15 S8/S16 R C1 C2 f(Hz)
0 0 0 0 0 0 1 1 548k 33n6 16n8 12
0 0 0 0 0 1 0 1 548k 29n4 14n7 14
0 0 0 0 1 0 0 1 548k 26n6 13n3 15
0 0 0 1 0 0 0 1 548k 24n4 12n2 17
0 0 1 1 0 0 0 1 492k 24n4 12n2 19
0 1 0 1 0 0 0 1 453k 24n4 12n2 20
1 0 0 1 0 0 0 1 420k 24n4 12n2 22
0 0 0 0 0 0 0 1 548k 20n 10n 21
0 0 1 0 0 0 0 1 492k 20n 10n 23
0 1 0 0 0 0 0 1 453k 20n 10n 25
1 0 0 0 0 0 0 1 420k 20n 10n 27
0 0 0 0 0 0 1 0 548k 13n6 6n8 30
0 0 1 0 0 0 1 0 492k 13n6 6n8 34
0 1 0 0 0 0 1 0 453k 13n6 6n8 37
1 0 0 0 0 0 1 0 420k 13n6 6n8 39
0 0 0 0 0 1 0 0 548k 9n4 4n7 44
0 0 1 0 0 1 0 0 492k 9n4 4n7 49
0 1 0 0 0 1 0 0 453k 9n4 4n7 53
1 0 0 0 0 1 0 0 420k 9n4 4n7 57
0 0 0 0 1 0 0 0 548k 6n6 3n3 62
0 0 1 0 1 0 0 0 492k 6n6 3n3 69
0 1 0 0 1 0 0 0 453k 6n6 3n3 75
1 0 0 0 1 0 0 0 420k 6n6 3n3 81
0 0 0 1 0 0 0 0 548k 4n4 2n2 93
0 0 1 1 0 0 0 0 492k 4n4 2n2 104
0 1 0 1 0 0 0 0 453k 4n4 2n2 113
1 0 0 1 0 0 0 0 420k 4n4 2n2 122

Construction

If you would like to make a high quality active low pass subwoofer filter like this for yourself, this section has everything you need to know!  I made mine using the the Press-n-Peel system developed by Techniks.  This is an easy system that can be used to print the circuit with a laser printer and literally iron it onto the copper board.  The complete process for this is described on my 8051 Microcontroller page or you can look at other Press 'n' Peel links on the web.  You will need two prints for the design, one for the copper tracks here, and one for the component side here.  These are pdf files that will print the images in the correct size.  Below you will find a list of components you will need and some example links of where you can get them from.  As there are so few components, I didn't bother numbering them R1, R2 etc. but it is easy to work out.  The numbers in brackets are the quantities required.  If you have any questions, let me know.

Tools Description Supplier
Anti-Static Mat Mat 23 x 27 Jameco 236540
Anti-Static Wristband Wristband Jameco 285456
Anti-Static Foam Conductive foam Jameco 13864
Soldering Iron 15 Watt Grounded Soldering Iron Radio Shack 64-2051
Spare Tip Spare Soldering Iron Tip Radio Shack 64-2052
Drill Mini Drill Jameco 26702
Drill Stand Mini Drill Stand Jameco 26711
Drill bit for components DRILL BIT,PC,SIZE 75 Jameco 16619
Drill bit for headers DRILL BIT,PC,SIZE 59 Jameco 16580
Board Supplies
PC Board Plain copper board 115mm x 160mm Radio Shack 276-1149
PCB Etchant Ferric Chloride Solution 16 Fl. Oz. Radio Shack 276-1535
Dry Ferric Chloride All Electronics CAT# ER-3
Dry Concentrated Etchant Jameco 70201
Press'n'Peel 5 Sheets of Press'n'Peel All Electronics CAT# TEK-5
Etch Resist Pen Resist Pen 416-RP ABRA Electronics 416-RP
Sticky Tape Highland Invisible Tape 6200 Office Depot
Goo Gone Toner Solvent Office Depot
Sticky back paper 1 Label per sheet Office Depot
Wire
Strap Wire Hookup wire 24 AWG Jameco 234168
Headers
Header Single Single row header, 22 pins Jameco 202825
Resistors
420K ohm (2) Jameco / DigiKey etc.
56K ohm (2)
39K ohm (2)
33K ohm (2)
1 ohm (2)
47K ohm (12)
10K ohm (2)
1k ohm (1)
1MK ohm (4)
Capacitors
2n2 (3) Jameco / DigiKey etc.
3n3 (3)
4n7 (3)
6n8 (3)
10n (3)
Chip Sockets
8 pin DIL (1) IC Socket Dual Wipe Solder Tail 8 Pin Jameco 259186
14 pin DIL (1) IC Socket Dual Wipe Solder Tail 14 Pin Jameco 112213
Chips
TL072 (1) Low noise JFET dual op amp Jameco 33195
TL074 (1) Low noise JFET quad op amp Jameco 33216
Switches
S1-S8 & S9-S16 (2) SWITCH, DIP, SPST, 8SWTH Jameco 38842

Pictures

Rev B Active Filter & and Mixer Board Rev B Active Filter & and Mixer Board Rev B Active Filter & and Mixer Board connecting to Velleman K4010 Solder side